When determining if a gene variant is associated with a genetic disorder, the variant is evaluated using scientific research to date, such as information on how the variant affects the function or production of the protein that is made from the gene and previous variant classification data.
The variant is then classified on a spectrum based on how likely the variant is to lead to the disorder. Evaluation needs to be done for each variant.
Just because a gene is associated with a disease, does not mean that all variants in that gene are pathogenic. Additionally, evaluation of a variant needs to be done for all diseases with which it is thought to be associated. A variant that is pathogenic for one disease, is not necessarily pathogenic for a different disease.
It is important to re-evaluate variants periodically; the classification of a variant can change over time as more information about the effects of variants becomes known through additional scientific research. Standards and guidelines for the interpretation of sequence variants: a joint consensus recommendation of the American College of Medical Genetics and Genomics and the Association for Molecular Pathology.
Genet Med. Epub Mar 5. Other chapters in Help Me Understand Genetics. And sometimes, although very rarely, the change in DNA sequence may even turn out to be beneficial to the organism.
A mutation that occurs in body cells that are not passed along to subsequent generations is a somatic mutation. A mutation that occurs in a gamete or in a cell that gives rise to gametes are special because they impact the next generation and may not affect the adult at all.
Such changes are called germ-line mutations because they occur in a cell used in reproduction germ cell , giving the change a chance to become more numerous over time. If the mutation has a deleterious affect on the phenotype of the offspring, the mutation is referred to as a genetic disorder. Alternately, if the mutation has a positive affect on the fitness of the offspring, it is called an adaptation.
Thus, all mutations that affect the fitness of future generations are agents of evolution. Mutations are essential to evolution. Every genetic feature in every organism was, initially, the result of a mutation. The new genetic variant allele spreads via reproduction, and differential reproduction is a defining aspect of evolution. It is easy to understand how a mutation that allows an organism to feed, grow or reproduce more effectively could cause the mutant allele to become more abundant over time.
Even deleterious mutations can cause evolutionary change, especially in small populations, by removing individuals that might be carrying adaptive alleles at other genes.
Hyla versicolor , is an example of mutation and its potential effects. When an ancestral Hyla chrysocelis gray treefrog failed to sort its 24 chromosomes during meiosis, the result was H. This treefrog is identical in size, shape and color to H.
All rights reserved. Most mutations occur at single points in a gene, changing perhaps a single protein, and thus could appear unimportant. For instance, genes control the structure and effectiveness of digestive enzymes in your and all other vertebrate salivary glands.
At first glance, mutations to salivary enzymes might appear to have little potential for impacting survival. Yet it is precisely the accumulation of slight mutations to saliva that is responsible for snake venom and therefore much of snake evolution.
Natural selection in some ancestral snakes has favored enzymes with increasingly more aggressive properties, but the mutations themselves have been random, creating different venoms in different groups of snakes. Snake venoms are actually a cocktail of different proteins with different effects, so genetically related species have a different mixture from other venomous snake families.
The ancestors of sea snakes, coral snakes, and cobras family Elapidae evolved venom that attacks the nervous system while the venom of vipers family Viperidae; including rattlesnakes and the bushmaster acts upon the cardiovascular system.
Both families have many different species that inherited a slight advantage in venom power from their ancestors, and as mutations accumulate the diversity of venoms and diversity of species increased over time. Although the history of many species have been affected by the gradual accumulation of tiny point mutations, sometimes evolution works much more quickly.
Several types of organisms have an ancestor that failed to undergo meiosis correctly prior to sexual reproduction, resulting in a total duplication of every chromosome pair. Such a process created an "instant speciation" event in the gray treefrog of North America Figure 2. The consequence of doubling the genome size in plants is often abnormally large seeds or fruits, a trait that can be of distinct advantage if you are a flowering plant! Most cereals that humans eat have enormous seeds compared to other grasses, and this is often due to the genomic duplications that occurred in the ancestors of modern rice and wheat and, because the mistake occurred in reproductive organs, was successfully passed on to future generations.
Humans themselves have mimicked this process by interbreeding individual plants with the largest fruits and seeds in the process of artificial selection, creating many of our modern agricultural crop strains.
Since all cells in our body contain DNA, there are lots of places for mutations to occur; however, not all mutations matter for evolution. For example, the yellow color on half of a petal on this red tulip was caused by a somatic mutation. The seeds of the tulip do not carry the mutation. Cancer is also caused by somatic mutations that cause a particular cell lineage e.
Such mutations affect the individual carrying them but are not passed directly on to offspring. The only mutations that matter to large-scale evolution are those that can be passed on to offspring. These occur in reproductive cells like eggs and sperm and are called germ line mutations.
This can happen in many situations: perhaps the mutation occurs in a stretch of DNA with no function, or perhaps the mutation occurs in a protein-coding region, but does not affect the amino acid sequence of the protein.
Imagine making a random change in a complicated machine such as a car engine. The chance that the random change would improve the functioning of the car is very small.
The change is far more likely to result in a car that does not run well or perhaps does not run at all. By the same token, any random change in a gene's DNA is likely to result in a protein that does not function normally or may not function at all. Such mutations are likely to be harmful. Harmful mutations may cause genetic disorders or cancer. What happens if a plant does not have chlorophyll? They would lack the part of the leaf that makes them green.
So these plants could be referred to as albino. This would have to result from a genetic mutation. Do these plants die because they cannot photosynthesize? Not necessarily. What can these plants tell us about the biochemistry, genetics and physiology of plants? Effects of Mutations The majority of mutations have neither negative nor positive effects on the organism in which they occur. Beneficial Mutations Some mutations have a positive effect on the organism in which they occur.
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